The invention relates generally to apparatus for recording and reading signals in tracks on an elongated tape such as a magnetic tape in the form of a ribbon, having substantially parallel longitudinal edges and having a certain resistance to bending in the plane of the tape and in particular to such an apparatus comprising: at least one magnetic head for recording and reading tracks on the magnetic tape; a tape guide member for guiding the magnetic tape in a path past the magnetic head (s), which tape guide member is provided with a tape-supporting edge for supporting one of the two longitudinal edges of the magnetic tape along the path between a tape-support entry point and a tape-support exit-point; means for sustaining a tape tension of a specific magnitude in the longitudinal direction of the magnetic tape; and pressure members for cooperating with, and exerting transverse pressure forces of a specific magnitude on, the magnetic tape in order to press the magnetic tape against the tape-supporting edge with a transverse force which is as constant as possible per unit of length over the entire part of the tape-supporting edge between the tape-support entry point and the tape-support exit point.
Such apparatus, generally referred to as magnetic tape recorders or briefly tape recorders, are known, for example, from U.S. Pat. No. 3,995,318 that relates to a helical-scan video recorder, i.e., a tape recorder for recording and reading video signals in obliquely directed adjacent tracks on a magnetic tape. The magnetic heads are rotatable about an axis of rotation in a circular path for recording and reading the oblique tracks on the magnetic tape. The magnetic heads are mounted on a magnetic head support in the form of a so-called "head disc", which is rotatable about the axis of rotation. The tape guide means comprises a tape guide drum system which is coaxial with the axis of rotation of the magnetic heads, which system has a circumferential surface around which the magnetic tape is guided by the tape-supporting edge in a substantially helical path along the circular path of the magnetic heads. The tape guide drum system comprises a stationary lower drum, on which the tape-supporting edge is located; the head disc; and a stationary upper drum. The head disc rotates with a small clearance between the upper drum and the lower drum. In the upper drum a plurality of axially directed slots are formed in which elongate wire springs are movable, which springs project over a small distance from the circumferential surface of the upper drum and bear on the longitudinal edge of the magnetic tape which is situated opposite the tape-supporting edge.
The purpose of the tape-supporting edge is to guide the magnetic tape in an accurately defined path along the tape guide means; i.e., in the aforementioned example, around the tape guide drum system. Deviations of the magnetic tape from this path result in tracks which have been recorded on the magnetic tape not being in the optimum position for reading by means of a magnetic read head. Especially when wide-band signals are recorded, such as video signals, it is very important that during read-out the recorded tracks move past the magnetic read head with great precision. If the magnetic tape, in its path past the magnetic heads, in addition to a constant deviation, also exhibits a deviation which varies in time, other effects occur which adversely affect the quality of the signal reproduced. Such fluctuations in the position of the magnetic tape on the tape guide means may, for example, occur owing to fluctuations in the shape of the longitudinal edges of the tape. Ribbon-like magnetic tapes are manufactured by cutting a wide strip of plastic foil, coated with a layer of a magnetic material, into ribbons by means of rotary cutters. During this cutting operation certain deviations in the shape of the longitudinal edges of the magnetic tapes are inevitable. During manufacture and during cutting of the ribbons the plastic foil is moreover subject to stresses which after cutting manifest themselves in the form of local changes in shape. It is therefore of importance that in the tape recorder the magnetic tape is pressed as firmly as possible against the tape-supporting edge in the transverse direction. It is obvious that there are limitations in this respect, because the magnetic tape is very thin and is therefore only capable of withstanding a small local load on its longitudinal edges. In the case of an excessive local load the tape is folded over locally. Therefore, it is important that locally the magnetic tape is not subjected to a load higher than a specific maximum permissible transverse force. The magnetic tape is pressed against the tape-supporting edge in an optimum manner when at any point of the tape-supporting edge the transverse force exerted by the magnetic tape equals the maximum permissible transverse force. Thus, it is important that over the entire portion of the tape-supporting edge between the tape-support entry point and the tape-support exit point a transverse force is exerted which is as constant as possible per unit of length.
Although in the foregoing the field to which the invention relates has been discussed with reference to a helical-scan video recorder, the invention is not limited to this field. Indeed, similar problems may occur in other equipment for recording and reading signals on a ribbon-like magnetic tape, for example if the magnetic tape passes along a straight tape guide member and the magnetic heads are not mounted for rotation but are stationary, such as in an audio magnetic tape recorder.